This invention relates generally to an apparatus for automatically preparing batches of a mixture containing several ingredients, and more particularly to an apparatus for automatically catalyzing and supplying resinous coatings for paperboard.
Coating, dipping and related operations are generally most efficient and cost-effective when done as continuous, on-line processes. Thus, efforts have been made to develop machines for automatically mixing ingredients and providing continuous supplies of coating to the line. In the case of resinous coatings for paperboard, this has been particularly difficult. Resinous coatings are extremely corrosive and will rapidly destroy any delicate mechanisms such as sensors which are in regular contact with the coating or its various components. Moreover, once the resinous coating is mixed its viscosity increases rapidly and it will harden into a glassy, unmanageable mass if left standing for any appreciable period. Even where this is not allowed to happen, deposits of hardened resin will tend to accumulate on the sides of the supply apparatus, creating shadow-readings on level probes and other sensors which operate by direct contact, and also rapidly clogging any bottlenecks, such as pipe bends or metering valves, in the apparatus. Thus it will be seen that a metered mix-control system such as the one described in U.S. Pat. No. 3,620,915 issued Nov. 16, 1971 to Keyes, IV et al, which requires contacting sensors and metering valves for flow control, would be unsuitable for use with resinous coatings.
The relative proportions of the ingredients in a resinous coating must be carefully controlled. For example, the catalyst rarely represents more than a few percent of the total solids content of the coating, but the addition of too much catalyst may cause the coating to solidify in the pipes of the supply system while the addition of too little may result in an unacceptably tacky coated web. While a resinous coating need contain only three ingredients, i.e. a concentrated resin, a catalyst, and water, small amounts of other ingredients such as defoamers, wetting agents, viscosity and pH modifiers, curing agents and scavengers are often added and it is desireable to be able to precisely control the amounts added, both to achieve optimum results and to control the costs of what are generally expensive ingredients.